A huge manhunt is underway across Europe for at least one suspect still at large after plotting Spain's terrorist attacks in which 14 people were killed and hundreds were injured.

The police are now hunting for Moroccan-born Younes Abouyaaqoub, said to be at the centre of the investigation into the Thursday massacre in which a van driven by terrorists ploughed into a crowd in Barcelona's Las Ramblas boulevard killing 13 civilians and injuring 13 people, the Telegraph reported.

Moussa Oukabir, the 18-year-old who was previously reported as the key suspect in Barcelona attack, was one of five men shot dead by police in a second terror attack in the coastal town of Cambrils, authorities said on Friday night.

Oukabir was killed along with terrorists Said Aallaa and Mohamed Hycham in the Cambrils attack that left one woman dead and five others injured after the men drove into pedestrians.

The other two terrorists killed in the attack are yet to be identified.

Police said that the suspects in the double attacks originally planned to use explosive devices to wreak greater devastation but were apparently thwarted because their materials detonated prematurely.

According to Spanish newspaper El Pais, police in Catalonia said they were searching for Abouyaaqoub, who is understood to be a key member of a 12-member jihadist cell behind the double attacks. The massive manhunt was launched amid fears that they could be preparing further attacks at popular tourist areas.

Abouyaaqoub, 22, lived in the town of Ripoll to the north of Barcelona. One former classmate described Abouyaaqoub as "very shy". She told La Vanguardia newspaper: "He didn't like attention.

"He was quiet and never got in trouble. It's very shocking to tell you the truth, here nobody can believe that he was capable of doing this," the classmate said.

So far, three people have been arrested in Ripoll and one person in Alcanar. Three of them are Moroccan citizens and another is Spanish, media reports said.

Authorities said there was a clear link between an explosion at a house at Alcanar on Wednesday and the attacks in Barcelona and Cambrils.